Nobel metal silicides and particularly iridium silicide can make excellent infrared sensitive Schottky barrier diodes which can be used in image sensors.
The solid phase of the iridium silicide should be formed free of contamination. The iridium silicide silicon interface is also critically important in obtaining high performance in these image sensors.
In U.S. Pat. No. 4,528,744 to Shibata, oxide regions are etched to reveal bare silicon substrate. Platinum metal is then deposited by an unspecified evaporation. The substrates are then annealed at 550.degree. C. for 15 minutes under unspecified ambient conditions to form platinum silicide. Unreacted platinum metal is then removed with an aqua regia etch.
It should be noted there is no known etchant like aqua regia which attacks and dissolves platinum metal that attacks iridium metal. The teachings of Shibata will not work for selectively forming iridium silicide and removing unreacted iridium metal. This invention may form the metal silicide which are contaminated by oxygen.
U.S. Pat. No. 4,109,372 to Geffken, uses a photoresist lift-off technique to form selective regions of platinum silicide and remove the unreacted platinum. First, an undercut photoresist profile is obtained and the underlying oxide regions is etched to reveal the bare silicon substrate. Next, the metal is "deposited by evaporative techniques or any other suitable method." Deposited metals that are disclosed are palladium, platinum, rhodium, and nickel.
At this point it should be noted that this process will not work under UHV (Ultra High Vacuum) conditions since photoresist is an organic material that would outgas in a UHV system and degrade its vacuum performance. After the evaporation the photoresist is removed using a photoresist solvent. This resist removal step lifts off the unreacted platinum metal that was deposited onto the top of the photoresist. The substrate is then annealed at 200.degree. C. to 550.degree. C. to react the platinum metal with the silicon substrate to form platinum silicide.